Dvc: Document new dir_helpers fixtures

[skshetry]

We are converting pytest style tests to use dir_helpers fixtures (ref: #2896).
It'd be great to document a basic usage of such fixtures, so, that, it'd take less effort for new contributors to start writing tests.

We can document it in conftest.py or dir_helpers.py itself, and small generic documentation on Contributing guide (that is on, dvc.org?).

[ghost]

@skshetry , isn't the module's docstring enough? I found it pretty useful)

Don't want to hijack this issue, but defining some general guidance on how to write tests would also be useful.